1. Field of the Invention
The present invention relates to a comparator by which it can be determined whether a certain voltage is in a predetermined range with respect to a reference voltage.
3. Description of the Prior Art
For the purpose of increasing the integration density, mask read-only memory cells having multi-values, for example, four values, recently have frequently been used. In the case of a cell having four values, the four values correspond to the voltage width or range of the transistor, and the mid value of the width is used as the value of the reference cell. The value of the reference cell is read out, and the readout value is coded into binary information. In such a cell having four values, the deviation of the width of the transistor of the reference cell should be small because the width of the transistor is originally small. Therefore, detection of the value of the reference cell becomes very difficult because the flip-flop circuit which is conventionally used cannot be used to detect this value.
A single flip-flop circuit can be used to determine whether or not an input voltage is larger or smaller than the reference voltage. However, a flip-flop circuit cannot be used to determine whether or not the input voltage is in the predetermined range with respect to the reference voltage or in which direction the input voltage is shifted from the predetermined range. In such a case, a window comparator comprising two comparators is used. However, a window comparator requires two reference voltages and the circuit of the window comparator is complicated, thereby rendering the comparator unsuitable for forming an integrated circuit.